Since the invention of the integrated circuit (IC), the semiconductor industry has experienced continued rapid growth due to continuous improvements in the integration density of various electronic components (i.e., transistors, diodes, resistors, capacitors, etc.). For the most part, this improvement in integration density has come from repeated reductions in minimum feature size, which allows more components to be integrated into a given area.
The past few decades have also seen many shifts in semiconductor packaging that have impacted the entire semiconductor industry. The introduction of surface-mount technology (SMT) and ball grid array (BGA) packages were generally important steps for high-throughput assembly of a wide variety of IC devices, while at the same time allowing for reduction of the pad pitch on the printed circuit board (PCB). Conventionally packaged ICs have a structure basically interconnected by fine gold wire between metal pads on the die and electrodes spreading out of molded resin packages. On the other hand, some chip scale packages (CSP) or BGA packages rely on bumps of solder to provide an electrical connection between contacts on the die and contacts on a substrate, such as a packaging substrate, a PCB, another die/wafer, or the like. In these cases, an under bump metal (UBM) layer is formed on a bond pad of the die, and a solder bump is placed on the UBM layer. The different layers making up this interconnection typically have different coefficients of thermal expansion (CTEs). As a result, a relatively large stress derived from this difference is exhibited on the joint area, which often causes cracks to form along the interface between the UBM layer and the solder bump.
In one attempt to reduce the stress between the solder bump and the UBM, the UBM was made larger. While this may reduce the stress in some devices, the increased size of the UBM caused the solder bump to become flat and deformed, thereby causing deformation issues and increasing the possibility of a bridging or shorting with adjacent solder bumps.